Control systems capable of generating an electronic pulse-width modulation signal for controlling electronic devices are well known. Such systems are commonly employed to control solenoids which are used to open and close valves in hydraulic systems.
When an electronic device is controlled through PWM, correction of the output PWM signal may be necessary, either due to fluctuations in power supply voltage or due to fluctuations in the inductance and resistance of the electronic device resulting from changes in temperature and valve spool position. This correction is commonly achieved through feedback control of the output PWM signal.
Feedback control is typically performed by determining average current flow through the electronic device, comparing the measured average current to a desired value, and adjusting the duty cycle of the PWM signal appropriately if the average current differs from the desired value.
Disadvantageously, and as will be described later in greater detail, both the manner in which the average current is determined and the manner in which the measured average current is compared to the input value may introduce error or delay into the output PWM signal.
Average current is typically determined by passing a voltage proportional to the current flowing through the device through an RC circuit which serves as a low-pass filter. The low-pass filter “smooths” the voltage to a DC value. Two problems may result from the use of such a low-pass filter. First, the output of the low pass filter can still have an AC component, resulting in a measured average current which fluctuates over time. The measured average current may therefore be erroneous depending upon the exact moment in time at which the value is sampled. Secondly, because the RC circuit introduces lag, a delay will exist between any input adjustments and a resultant change in the output PWM signal. Moreover, this delay will be variable depending upon the PWM signal frequency. This is due to the fact that the RC circuit has a fixed cut-off frequency. When the output PWM frequency is changed, the number of degrees out-of-phase between the input signal and the output signal will also change.
Comparison of the measured average current (i.e. output of the low-pass filter) to the input value is typically performed using a proportional-integral (PI) error amplifier. This component also disadvantageously introduces a delay between any input adjustments and resultant output PWM signal changes.
What is needed is a solution which addresses at least some of these difficulties.